Post-collision, Shoshonitic Volcanism on the Tibetan Plateau: Implications for Convective Thinning of the Lithosphere and the Source of Ocean Island Basalts   总被引：95，自引：1，他引：94  

TURNER  S.; ARNAUD  N.; LIU  J.; ROGERS  N.; HAWKESWORTH  C.; HARRIS  N.; KELLEY  S.; VAN CALSTEREN  P.; DENG  W. 《Journal of Petrology》1996,37(1):45-71

Potassic volcanism has been widespread and semi-continuous onthe Tibetan plateau since 13 Ma, post-dating the orogenic thickeningof the India-Asia collision. Volcanism may have commenced slightlyearlier (16–20 Ma) in the southern portion of the plateauand then ceased around 10 Ma. The dominant lavas are pyroxen-and plagioclase-phyric shoshonites with subordinate occurrencesof dacites and rhyolites. Their mineralogy reflects crystallizationfrom high-temperature (1100C) magmas which had elevated oxygenand water fugacities. Geochemically, they are characterizedby relatively low TiO², Al²O³ and Fe²O³, and high Na²O, coupledwith variable abundances of compatible trace elements and veryhigh contents of incompatible trace elements. Normalized incompatibleelement patterns have marked negative Nb, Ta and Ti anomalieswhereas K²O appears to be buffered at 4% over a wide range ofSiO². Isotope data show a relatively broad and enriched rangeof ⁸⁷Sr/⁸⁶ Sr (0.7076–0.7106) at more restricted ENd (–5.2to –8.1). Pb isotopes are characterized by a range of²⁰⁷Pb/²⁰⁴ Pb (15.51–15.72) and ²⁰⁸ Pb/²⁰⁴Pb (38.67–39.30) at very uniform ²⁰⁶Pb/²⁰⁴ Pb (18.39–18.83), leadingto vertical arrays. Volcanics from the southern parts of theplateau have more primitive isotopic compositions: ⁸⁷Sr/⁸⁶ Sr0.7048–0.7080, _Nd 1.4 to –3.3, ²⁰⁶Pb/²⁰⁴ Pb 18.48–18.67,²⁰⁷Pb/²⁰⁴ Pb 15.59–15.68 and ²⁰⁸Pb/²⁰⁴ Pb 38. 73–38.98. In general, the geochemical and isotopic data most closely approximatepartial melting arrays, although fractionation processes haveclearly operated. The isotopic ratios and the enrichment ofincompatible elements and LREE/HREE cannot be derived from adepleted mantle source via a single-stage melting process. Instead,a metasomatized, garnet peridotite source containing 6% phlogopiteis required and this is inferred to lie within the lithosphericmantle. The enrichment in incompatible elements in this sourcemust have been sufficiently ancient to generate the observedisotopic ratios, and Nd depleted mantle model ages suggest thiswas Proterozoic in age (1.2 Ga), whereas Pb model ages recordan Archaean event, suggesting the source had a multi-stage enrichmenthistory. The negative Ta, Nb and Ti anomalies and low Rb/Basuggest that metasomatism may have occurred during an ancientsubduction episode. The high ²⁰⁸Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴ Pb and lowNb/U, Ce/Pb of the Tibetan shoshonites are distinct from oceanisland basalts. Thus, if convectively removed lithospheric mantleprovides a source for ocean island basalts, it must differ significantlyfrom the source of the Tibetan shoshonites. A lithospheric mantle source for the volcanism places importantconstraints on geodynamic models for the evolution of the Tibetanplateau and the India-Asia collision. For likely thermal structuresbeneath the plateau, the temperatures required to trigger meltingwithin the lithospheric mantle can only be plausibly obtainedif the lower parts of the lithospheric mantle were removed byconvective thinning. This is consistent with recent models whichinvoke the same process to explain the current elevation andextensional deformation of the plateau. The age data suggestthis occurred at 13 Ma and the duration of volcanism may beexplained by continued conductive heating since that time. Poorlysampled but slightly older volcanics from the southern portionsof the plateau may indicate that convective thinning began inthe south and migrated northwards. Rapid uplift of the plateaumay have resulted in increased rates of chemical weathering,which led to global cooling, as indicated by oxygen isotopedata from Atlantic sediments. KEY WORDS: Climate; lithospheric mantle; OIB; Tibet; volcanism ^*Corresponding author.  相似文献   

Emplacement of Deep Upper-Mantle Rocks into Cratonic Lithosphere by Convection and Diapiric Upwelling     

DRURY  M. R.; VAN ROERMUND  H. L. M.; CARSWELL  D. A.; DE SMET  J. H.; VAN DEN BERG  A. P.; VLAAR  N. J. 《Journal of Petrology》2001,42(1):131-140

Rocks containing breakdown products of majoritic garnet, derivedfrom the deep upper mantle, occur in kimberlite xenoliths andin orogenic peridotites from Otrøy in Norway. The Otrøyperidotites are banded harzburgites and dunites with similarcompositions to mantle xenoliths from Precambrian cratons andPhanerozoic supra-subduction-zone peridotites. Pressure–temperature(P–T) paths deduced for the Otrøy peridotites andkimberlite xenoliths from South Africa are consistent with emplacementof deep mantle peridotites into cratonic lithosphere by asthenospherediapirism. Numerical thermo-convection models provide insightinto the possible P–T histories of deep upper-mantle rocks.In the models, material from the base of the convecting systemis transported to depths of 60–100 km by convection andsmall (50–100 km) diapirs. Diapir intrusion induces small-scaleconvection in the low-viscosity deeper part of the thermochemicallydefined lithosphere. Small-scale convection in the craton rootcan produce complex P–T paths, complex recurrent meltinghistories and complex compositional structure in the craton.P–T paths derived from the numerical models for asthenospherediapirism in a hot upper mantle are consistent with the sequenceof sub-solidus P–T conditions deduced for the cratonicperidotites. KEY WORDS: asthenosphere diapirs; cratonic lithosphere; deep upper mantle; majoritic garnet  相似文献   

MANUFACTURING ACTIVITIES IN FOUR TOWNS IN CENTRAL JAVA: AN ASSESSMENT OF THEIR DEVELOPMENT POTENTIAL     

M.J. TITUS  A.A. VAN DER WOUDEN 《Singapore journal of tropical geography》1993,13(2):130-151

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RESOLUTION ANALYSIS OF BAND-LIMITED AND OFFSET-LIMITED SEISMIC DATA FOR PLANE-LAYERED SUBSURFACE MODELS1     

E. P. F. VAN RIJSSEN  G. C. HERMAN 《Geophysical Prospecting》1991,39(1):61-76

A singular-value decomposition technique is presented for quantifying the information content of band-limited and offset-limited seismic reflection data for the case of plane-layered subsurface models. With the aid of this method, vertical resolution, tuning effects and ambiguities between different types of lithological parameters can be analysed simultaneously. The method is applied in a model study for quantifying the effects of offset-limitation and the presence of different wavetypes on the ability to discriminate different types of parameters.  相似文献   

Lp-NORM DECONVOLUTION1     

H. W. J. DEBEYE  P. VAN RIEL 《Geophysical Prospecting》1990,38(4):381-403

The purpose of deconvolution is to retrieve the reflectivity from seismic data. To do this requires an estimate of the seismic wavelet, which in some techniques is estimated simultaneously with the reflectivity, and in others is assumed known. The most popular deconvolution technique is inverse filtering. It has the property that the deconvolved reflectivity is band-limited. Band-limitation implies that reflectors are not sharply resolved, which can lead to serious interpretation problems in detailed delineation. To overcome the adverse effects of band-limitation, various alternatives for inverse filtering have been proposed. One class of alternatives is L_p-norm deconvolution, L₁norm deconvolution being the best-known of this class. We show that for an exact convolutional forward model and statistically independent reflectivity and additive noise, the maximum likelihood estimate of the reflectivity can be obtained by L_p-norm deconvolution for a range of multivariate probability density functions of the reflectivity and the noise. The L_∞-norm corresponds to a uniform distribution, the L₂-norm to a Gaussian distribution, the L₁-norm to an exponential distribution and the L₀-norm to a variable that is sparsely distributed. For instance, if we assume sparse and spiky reflectivity and Gaussian noise with zero mean, the L_p-norm deconvolution problem is solved best by minimizing the L₀-norm of the reflectivity and the L₂-norm of the noise. However, the L₀-norm is difficult to implement in an algorithm. From a practical point of view, the frequency-domain mixed-norm method that minimizes the L₁norm of the reflectivity and the L₂-norm of the noise is the best alternative. L_p-norm deconvolution can be stated in both time and frequency-domain. We show that both approaches are only equivalent for the case when the noise is minimized with the L₂-norm. Finally, some L_p-norm deconvolution methods are compared on synthetic and field data. For the practical examples, the wide range of possible L_p-norm deconvolution methods is narrowed down to three methods with p= 1 and/or 2. Given the assumptions of sparsely distributed reflectivity and Gaussian noise, we conclude that the mixed L₁norm (reflectivity) L₂-norm (noise) performs best. However, the problems inherent to single-trace deconvolution techniques, for example the problem of generating spurious events, remain. For practical application, a greater problem is that only the main, well-separated events are properly resolved.  相似文献   

Determination of rasterizing error a case study with the soil map of The Netherlands     

A. K. BREGT  J. DENNEBOOM  H. J. GESINK  Y. VAN RANDEN 《International journal of geographical information science》2013,27(3):361-367

Abstract

The digital soil map of The Netherlands (scale 1: 50 000) in vector form was rasterized using three sizes of raster cell and two different rasterizing methods. The rasterizing errors were estimated for several map sheets of different complexity using the double-conversion method, the Switzer method and the Goodchild method. The relationship between the complexity of the maps, indicated by the boundary index, and the rasterizing error is presented as a regression equation. The double-conversion method provided a better estimation of the rasterizing error than the other two methods. Differences of less than 1 per cent were found between the rasterizing methods used.  相似文献   

Integrated micro- and macro-scale analyses of Last Glacial Maximum Irish Sea Diamicts from Abermawr and Traeth y Mwnt, Wales, UK     

JOHN F. HIEMSTRA  KENNETH F. RIJSDIJK  DAVID J. A. EVANS  JAAP J. M. VAN DER MEER 《Boreas: An International Journal of Quaternary Research》2005,34(1):61-74

This study is part of a basin-wide re-evaluation of Irish Sea glacigenic deposits which aims to test whether diamicts, collectively known as Irish Sea Tills, represent in situ glacimarine sediments or sediments that have been reworked or deformed by the Last Glacial Maximum Irish Sea Glacier. New results are presented for two key localities at Abermawr and Traeth y Mwnt in Wales. Unlike previous studies in the Irish Sea region that have focused on macro-scale sedimentology and structural analyses, this study combines macro-scale and micro-scale sedimentary analyses. This approach reveals that the dominant diamict facies at Abermawr are subglacially deformed primary (glaci)marine deposits, emplaced by the Irish Sea Glacier. An inland glacial source is unlikely. The Traeth y Mwnt diamicts are likely to be subaqueous in origin, possibly formed in an ice-dammed lake in the Mwnt embayment. There are no indications of subglacial deformation or shearing at Mwnt; deformation structures are related to gravity-driven or density-driven mechanisms.  相似文献   

Significant change of extratropical natural variability and potential predictability associated with the El Niño/Southern Oscillation     

WILBUR Y. CHEN  HUUG M. VAN DEN DOOL 《地球，A辑:动力气象学与海洋学》1999,51(5):790-802

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Pleistocene sandur deposits represent braidplains, not alluvial fans     

TOMASZ ZIELINSKI  ANTONIUS J. VAN  LOON 《Boreas: An International Journal of Quaternary Research》2003,32(4):590-611

Weichselian sandar in NE Poland show characteristics that are inconsistent with the commonly accepted alluvial-fan-like model for outwash deposition and sandur formation. Analysis of the lithofacies and their vertical and lateral transitions indicates that the Polish sandar developed as braidplains, not as alluvial fans. Analysis of the geomorphic conditions under which modern sandar form, indicates that these conditions (which are characterized by deposition in a narrow belt between ice-covered mountain ranges and the sea) cannot be considered representative of those that prevailed in the geological past when sandar developed as braidplains in confined valleys, to end up in a lowland area where the deposits could spread out further in lateral directions. The latter conditions have been found consistently for all Polish Weichselian sandar that were investigated in much sedimentological detail. This raises the question whether sandar are alluvial fans or not. Because the development of the sandar in NE Poland seems to be much more representative for outwash deposition than the present-day sandar in Iceland and elsewhere, the current alluvial-fan-like sandur model - based on the fairly exceptional present-day situation with deposition in a narrow belt - should therefore be replaced by the braidplain-like sandur model - based on deposition in a valley and in a wider lowland area in front - that has been established for the Polish examples.  相似文献   

THE LATERAL INFLOW INTO SUBMERGED DRAINS     

A. VAN DER BEKEN  F. DE TROCH  M. DE SOMER  F. C. ZUIDEMA 《水文科学杂志》2013,58(3):273-289

ABSTRACT

An attempt is made to explain the field measurements of piezometric height and discharge rate in a submerged drain system. The lateral inflow into the drain pipe is not necessarily uniformly distributed as is usually assumed. Hence, in analysing the hydrological performance of the drain pipe in the field, this fact must be considered. A general formula (equation (11)) is presented for calculating the actual distribution of the lateral inflow and a practical application is discussed.  相似文献